Symmetry of isoscalar matrix elements and systematics in the sd and beginning of fp shells

A careful determination of the lifetime and measurement of the branching ratio for decay of the first 2 T=1 + state in 42Sc has allowed an accurate experimental test of charge independence in the A = 42 isobaric triplet. A lifetime of 69(17) fs was measured at the University of Kentucky, while relative intensities for the 975 keV and 1586 keV transitions depopulating the first 2 T=1 + state have been determined at the University of Cologne as 100(1) and 8(1), respectively. Both measurements give an isoscalar matrix element, M 0, of 6.4(9) (W.u.) 1/2. This result confirms charge independence for the A=42 isobaric triplet. Shell model calculations have been carried out for understanding the global trend of M 0 values for A = 4n + 2 isobaric triplets ranging from A = 18 to A = 42. The 2 1(T=1) +→0 1(T=1) + transition energies, reduced transition probabilities and M 0 values are reproduced to a high degree of accuracy. The trend of M 0 strength along the sd shell is interpreted in terms of the shell structure. Certain discrepancies arise at the extremes of the sd shell, for the A = 18 and A = 38 isobaric triplets, which might be explained in terms of the low valence space at the extremes of the sd shell. © 2006 American Institute of Physics.